1. Field of the Invention
The present invention relates to a middle or large scale sealed metal oxide/metal hydride battery of more than 10 Ah having excellent charging characteristics at high temperatures and excellent reliability which comprises a positive electrode of a metal oxide which has a high capacity and is light in weight and a negative electrode of a hydrogen storage alloy having a high capacity and a lower equilibrium hydrogen gas pressure.
2. Description of the Related Art
Attention has been focused on an electrode of a hydrogen storage alloy capable of reversibly absorbing and desorbing hydrogen about at normal temperature under normal pressure as a new negative electrode for use in alkaline storage batteries because it permits charging and discharging and in addition it is superior in energy density to the cadmium electrode known for many years.
Particularly, during the last about ten years, intensive research has been conducted vigorously on such electrodes in the field of cell and battery industry. As a result of the research and development, hydrogen storage alloy electrodes have been employed in commercially available high capacity secondary cells which have started to be used practically in various handy electronic appliances.
These cells are mainly of a miniaturized sealed cell type, typically a cylindrical sealed type, and nickel-hydrogen storage batteries with the positive electrode being nickel and the negative electrode being a hydrogen storage alloy.
They generally have a structure where both a nickel positive electrode and a negative electrode of a hydrogen storage alloy are rolled into a spiral form with a resin separator being interposed between the positive electrode and the negative electrode, the roll being inserted into a cylindrical metal can and sealed with a lid equipped with a salty vent.
As the nickel positive electrode, there is used a sintered electrode which is manufactured by impregnating a sintered nickel substrate as used generally in the nickel-cadmium cells with a solution of a nickel salt and then converting the salt into nickel hydroxide, or an electrode which is manufactured by mixing a powder containing a major component of nickel hydroxide with a small amount of additives such as cobalt and cobalt oxide into a paste, filling the paste into a high porosity electrode substrate consisting of a bulk nickel sponge, and then drying, followed by subjecting to a molding under pressure (referred to as a SME type hereunder; These are disclosed in U.S. Pat. Nos. 4,251,603 and 4,935,318, and Japanese Patent KOKAI (Laid-open) Nos. 56-102076 and 55-39179).
As the hydrogen storage alloy negative electrode, on the other hand, there are known (1) an electrode which is manufactured by filling a powdery hydrogen storage alloy such as a mish-metal of a LnNi5 system, where Ln represents one of the rare earth elements having an atomic number from 57 to 71 or a mixture thereof (referred to as Mm hereunder) into a substrate of a bulk porous nickel sponge (as disclosed in U.S. Pat. No. 4,935,318), (2) an electrode which is manufactured by applying the powdery alloy to a plane perforated plate with an adhesive, (3) an electrode which is manufactured by applying a powdery Ti based alloy of a TiNi system (as disclosed in U.S. Pat. No. 4,849,205) to a plane perforated plate with an adhesive, and (4) an electrode which is manufactured by applying the above, followed by sintering. As other hydrogen storage alloy materials, there have been proposed a Zr based alloy of a ZrNi system which is promising for use as electrodes from the standpoint of high capacity (as disclosed in U.S. Pat. No. 4,946,646).
These negative electrodes are used in combination with any one of the positive electrodes described above and rolled in a spiral form with a separator being interposed therebetween. The capacity of the negative electrode is generally larger than that of the positive electrode.
As the separator materials, there are normally used non-woven fabrics made of polyamide resin fibers which are used generally in the nickel-cadmium cells, though the use of non-woven fabrics made of sulfonated polyolefin resin fibers which have a remarkable effect in reduction of self-discharge has been proposed already as disclosed in U.S. Pat. Nos. 4,837,119 and 5,100,723 and Japanese Patent KOKAI (Laid-open) No. 64-57568.
The electrolyte to be used is principally a solution of caustic potash identical to that used in the nickel-cadmium cells.
In this type of cell, if the partial pressure of hydrogen in the cell is reduced below the equilibrium hydrogen gas pressure of the hydrogen storage alloy used, the alloy releases hydrogen gas. Therefore, if the cell is used open, the amount of the electrode decreases due to the escape of the gas causing a deterioration of the cell performance and an increase in the internal cell pressure.
Therefore, a sealed cell equipped with a safety vent should be constituted. The pressure at which the safety vent of the cell is operated is determined by taking into consideration an increase in the internal cell pressure due to oxygen and hydrogen at the time of overcharging and an increase in the equilibrium hydrogen gas pressure when a reaction of gaseous oxygen and hydrogen proceeds accompanied by an increase in temperature.
The cells which have been proposed until now are mainly of miniaturized cylindrical sealed type and are excellent in resistance to pressure at the locations of the container and the seal. Therefore, even if an overcharging at such a large current as 1 C (the cell is charged at one hour rate) occurs, the tightness can be retained allowing a higher pressure for operating the safety vent to be employed. The pressure opening the vent to be used is generally set in the range of 10 to 30 kg/cm.sup.2. This is partly attributable to a great ability of dissipating heat, as in a small type cell.
Description has been made heretofore on the outline of the prior art techniques as to miniaturized nickel-hydrogen storage batteries which have started to be put into practical use already.
In recent years, however, a vigorous need has been created for high energy density cells having high reliability and a medium or great capacity as portable supplies to be used in a wide variety of applications such as domestic electrical appliances, electric automobiles and the like.
The metal oxide-hydrogen storage batteries are considered to be of a type of cell promising for the need. As the metal oxide, nickel oxide and manganese oxide are suitable from the economical point of view. Some attempts have been made to manufacture a nickel-hydrogen storage battery using nickel oxide as reported. However, few practical cell structures have been proposed with respect to a medium capacity cell (defined here as a capacity of 10 Ah to 100 Ah) and a large capacity cell (defined here as a capacity no less than 100 Ah) at present.
Attention is directed to the nickel-hydrogen storage battery as being of a typical cell type representing the metal oxide-hydrogen storage batteries. There are many technical problems to be overcome in modifying the small cells of this type to have a medium or large capacity while retaining the characteristics of the small cells, which problems will be discussed hereunder.
1. Enhancement in energy density and high reliability
Particularly, when the storage batteries are used as portable supplies, great interest is directed to increasing the energy density per unit weight (Wh/kg) as opposed to the small cells. Therefore, it is important to use electrodes having a lighter weight and a higher capacity per unit volume in combination. However, in the medium or large capacity cells, an increase in temperature owing to poor heat dissipation and high temperature atmosphere caused during operation diminishes the charge acceptability of conventional positive electrodes, if employed as they are, to inhibit attainment of desired energy density. Moreover, in order to obtain a high reliability long life cell, an improvement of electrode structure is more required to prevent dislodging of positive and negative electrode materials than is required for the small cells.
2. Structure of sealed cells
Sealed medium or large capacity cells are difficult to manufacture in the cylindrical form because of their high capacity as opposed to the small cells. For this reason, a square form is preferably employed, though it causes generally a significant reduction in the resistance to pressure of the container. It is necessary, therefore, to suppress the internal cell pressure at a smaller magnitude than the conventional one even at the last stage of charging, at which the internal cell pressure is the highest in the normal operation of the cell. In addition, a structure for imparting durability to sealings such as poles is necessary to employ.
3. Lower self-discharge
This cell type is said to undergo seriously great self-discharge as compared to the nickel-cadmium cells. When such highly self-discharging cells are employed in the applications where many opportunities to use them at high temperatures or to leave them unattended occur, they undergo a serious reduction in capacity so that the practical use of them may be hindered. Therefore, the self-discharge must be lowered.
4. Establishment of safety
Generally, there has been an increasing risk of the exotherm caused by shorting between the positive electrode and the negative electrode or of the explosion caused by a reaction between gaseous oxygen and hydrogen evolving inside the cell as the capacity of the cell is increased to a medium or high level. Therefore, there is a need for development of a highly safe cell by arranging a cell structure which is difficult to cause short-circuit, or does not cause any ignition even if a short-circuit occurs, or does not allow the gaseous oxygen and hydrogen to contact any source of ignition.
As above, there are many technical problems to be overcome in order to allow the nickel-hydrogen storage battery with a hydrogen storage alloy having a capacity ranging from a low level to a medium or high level to find a wide variety of practical application.
The present invention intends to overcome the aforementioned problems. That is, it is an object of the present invention to provide a sealed metal oxide-hydrogen storage battery having a higher energy density (especially, in the terms of Wh/kg) and a high reliability in a wide range from a higher temperature to a lower temperature, having excellent safety with regard to strength and evolution of internal gases, and a lower level of self-discharge.